Since polyester has a high melting point and high oxidation resistance, and further has low hydrophilicity, it is useful as a material of a separator for a nonaqueous electrolyte battery. However, on the other hand, a separator made of polyester hydrolyzes in a basic condition, and therefore the separator has a defect that a battery resistance increases due to the hydrolysis of the separator when the separator is used together with an active material containing a large amount of residual alkali components.